Switch over state in `HTTPConnectionPool.HTTP2StateMachine.failedToCreateNewConnection`

[dnadoba]

Current code assumes that we are always in the running state. We therefore never transition from the shuttingDown to the shutDown state if the last connection in the pool fails.

async-http-client/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift

Lines 400 to 407 in 9937d87

	mutating func failedToCreateNewConnection(_ error: Error, connectionID: Connection.ID) -> Action {
	self.failedConsecutiveConnectionAttempts += 1
	self.lastConnectFailure = error
	let eventLoop = self.connections.backoffNextConnectionAttempt(connectionID)
	let backoff = calculateBackoff(failedAttempt: self.failedConsecutiveConnectionAttempts)
	return .init(request: .none, connection: .scheduleBackoffTimer(connectionID, backoff: backoff, on: eventLoop))
	}

We likely need to do something similar to the HTTP1StateMachine.

async-http-client/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1StateMachine.swift

Lines 216 to 242 in 9937d87

	mutating func failedToCreateNewConnection(_ error: Error, connectionID: Connection.ID) -> Action {
	self.failedConsecutiveConnectionAttempts += 1
	self.lastConnectFailure = error
	switch self.lifecycleState {
	case .running:
	// We don't care how many waiting requests we have at this point, we will schedule a
	// retry. More tasks, may appear until the backoff has completed. The final
	// decision about the retry will be made in `connectionCreationBackoffDone(_:)`
	let eventLoop = self.connections.backoffNextConnectionAttempt(connectionID)
	let backoff = calculateBackoff(failedAttempt: self.failedConsecutiveConnectionAttempts)
	return .init(
	request: .none,
	connection: .scheduleBackoffTimer(connectionID, backoff: backoff, on: eventLoop)
	)
	case .shuttingDown:
	guard let (index, context) = self.connections.failConnection(connectionID) else {
	preconditionFailure("Failed to create a connection that is unknown to us?")
	}
	return self.nextActionForFailedConnection(at: index, context: context)
	case .shutDown:
	preconditionFailure("The pool is already shutdown all connections must already been torn down")
	}
	}